Image reading apparatus

ABSTRACT

An image reading apparatus includes image sensors for reading images on the front and back faces of an original, a buffer RAM for storing first image data representing the image on the front face of the original, and second image data representing the image on the back face of the original, which data are output from the image sensors, and an output circuit for selecting and outputting one of the first and second image data read out from the buffer RAM.

This application is a continuation of application Ser. No. 08/045,298,filed Apr. 12, 1993, abandoned, which is a division of application Ser.No. 07/766,020 filed Sep. 26, 1991, U.S. Pat. No. 5,392,135.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image reading apparatus and, moreparticularly, to an image reading apparatus for continuously carryingsheet-like originals, and reading one or both faces of original imagesusing image sensors arranged along a carrying path.

2. Related Background Art

In recent years, various image data file systems have been proposed. Inan image data file system of this type, image data is stored in a filemedium such as an optical disk or an optomagnetical disk, the storedimage data is retrieved and read out as needed, and the readout imagedata is reproduced, i.e., printed or displayed.

In order to deal with a dual-face original, the file system comprises anoriginal carrying/reading mechanism. In this mechanism, a singleoriginal is carried twice, so that its front face is read first, theoriginal is then reversed, and thereafter, its back face is read.Alternatively, the mechanism is provided with a carrying path having alength twice or more the length of an original to be read, so that thefront face of an original is read in the first half of the carryingpath, and the back face of the original is then read in the second halfof the carrying path.

In the above-mentioned arrangement, images on both the faces can beread. However, in the former twice-carrying type mechanism, in order toread a dual-face original, the original must be manually inserted twice,or a mechanism for automatically inserting the original in a readingunit arranged along the carrying path twice must be arranged. In eithercase, the sheet insertion operation requires a time twice or more thatof a conventional sheet insertion operation. When the mechanism forautomatically inserting an original twice is arranged, the arrangementof the original carrying path is complicated, and this leads to not onlyan increase in manufacturing cost of the system but also an increase infrequency of original jamming.

On the other hand, since the latter single-carrying type mechanismrequires the carrying path having a length twice or more that of aconventional mechanism, the sheet insertion operation requires a timetwice or more that of a conventional sheet insertion operation, and thesize of the system itself undesirably becomes bulky.

Thus, an image reading apparatus, in which two image sensors (solidstate image pickup elements such as CCD (charge-coupled device) linesensors) are arranged above and below a carrying path for carrying asheet-like original as means for easily inputting original data on adual-face original at high speed, and images on both the front and backfaces of the dual-face original are continuously read within one scanline at the same time to perform the subsequent image processing, hasbeen put into practical applications.

However, the above-mentioned image reading apparatus reads two faces ofan original which has effective image data on only one face, and imagedata on an unnecessary face is stored in a file medium such as anoptical disk or an optomagnetical disk, thus considerably impairingstorage efficiency of a file device.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the abovesituation, and has as its object to provide an image reading apparatuswhich can efficiently execute both a dual-face image reading operationand a single-face image reading operation of an original at high speed.

It is another object of the present invention to provide an imagereading apparatus which can efficiently execute a single-face imagereading operation of an original using a mechanism for performing adual-face image reading operation of an original.

It is still another object of the present invention to provide an imagereading apparatus suitable for an electronic image file.

It is still another object of the present invention to provide an imagereading apparatus which can read an original image at high speed with acompact arrangement.

The above and other objects and effects of the present invention willbecome apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a circuit arrangement of an imagereading apparatus according to the first embodiment of the presentinvention;

FIG. 2 is a sectional view showing in detail an image reading unitaccording to the embodiment of the present invention;

FIG. 3 is an explanatory view showing a reading output state of an imagesensor;

FIG. 4 is a block diagram showing a circuit arrangement of an imagereading apparatus according to the second embodiment of the presentinvention;

FIG. 5 is a schematic block diagram showing a circuit arrangement of animage reading apparatus according to the third embodiment of the presentinvention;

FIG. 6(A) to 6(F) are timing charts showing a drive timing of a CCDimage sensor in a dual-face reading mode;

FIG. 7(A) to 7(F) are timing charts showing a drive timing of the CCDimage sensor in a single-face, twice-speed reading mode;

FIG. 8(A) to 8(F) are timing charts showing a drive timing of the CCDimage sensor in a single-face, initial reading mode;

FIG. 9 is a schematic block diagram showing a circuit arrangement of animage reading apparatus according to the fourth embodiment of thepresent invention;

FIG. 10 is a schematic block diagram showing a circuit arrangement of animage reading apparatus according to the fifth embodiment of the presentinvention;

FIG. 11 is a schematic block diagram showing a circuit arrangement of animage reading apparatus according to the sixth embodiment of the presentinvention; and

FIG. 12(A) to 12(D) are waveform charts for explaining a change state ofa pixel signal output of the image reading apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be described indetail below with reference to the accompanying drawings.

First Embodiment

FIG. 1 shows a basic circuit arrangement according to the firstembodiment of the present invention. An image reading apparatus shown inFIG. 1 comprises CCD image sensors 1a and 1b for simultaneously readingimages on the front and back faces of a carried original, a mixingcircuit 2 for mixing analog pixel data from the CCD image sensors 1a and1b to obtain one signal, and an image binarization circuit 3 for A/D(analog-to-digital)-converting analog pixel data from the mixing circuit2 into digital multi-value pixel data, performing digital imageprocessing necessary for improving image quality, and binarizing thedata using an internal digital comparator.

The image reading apparatus further comprises an input switching circuit4 for selecting one of two binary image data, a front & back imagemanagement circuit 12 for receiving an output signal from the inputswitching circuit 4, and selecting whether front & back batch image datais output as an image to be output to the next stage, or front- orback-face image data is selectively output in accordance with aninstruction from an operating panel 13, an image data compressioncircuit 5 for performing image compression by encoding binary image datafrom the front & back image management circuit 12, an image dataexpansion circuit 6 for decoding the encoded image data to binary imagedata, an image buffer RAM 7 for temporarily storing the encoded imagedata, a transfer control circuit 8 for performing transfer control withan SCSI (small computer system interface) bus, an SCSI bus 9 for finallyoutputting image data, and an image data transfer control circuit (DMAtransfer control circuit) 10 for performing image data transfer controlamong the respective devices.

FIG. 2 shows in detail a reading unit applied to the embodiment shown inFIG. 1. A dual-face original 25 which is carried by two pairs ofcarrying rollers 20a, 20b, 20c, and 20d along a path between carryingguides 22a and 22b is read by reading units arranged at substantiallythe same physical position. An image (reflected light) on the front faceof the original 25 is projected and focused on the CCD image sensor 1afor use in reading of the front face along an optical path including areading glass 21a, a total reflection plate 23a, and a focusing lens24a. On the other hand, an image (reflected light) on the back face ofthe original 25 is projected and focused on the CCD image sensor 1b foruse in reading of the back face along an optical path including areading glass 21b, a total reflection plate 23b, and a focusing lens24b.

Referring back to FIG. 1, the operations of the apparatus of thisembodiment will be explained below. Output timings of pixel data read bythe two CCD image sensors 1a and 1b are controlled by a CCD transferdrive circuit (not shown), so that data from the CCD image sensor 1a foruse in reading of the front face is output in the first half of one scanline period, and data from the CCD image sensor 1b for use in reading ofthe back face is output in the second half of the period. The analogpixel data output from the two CCD image sensors 1a and 1b are mixed bythe mixing circuit (analog switch) 2, as shown in FIG. 3, thus obtaininga single signal line including front-face image data in its first half,and back-face image data in its second half within one scan line.

The image binarization circuit 3 A/D-converts the analog pixel datamixed by the analog switch 2 into digital multi-value pixel data,performs digital image processing necessary for improving image quality,and then performs binarization processing of the pixel data.Subsequently, the binarized pixel data is encoded by sequentialprocessing of the image data compression circuit 5 via the inputswitching circuit 4 switched to the image binarization circuit 3 side,and the front & back image management circuit 12 switched to a front &back batch output state. The encoded data is input to the image bufferRAM 7 under the control of the image data transfer control circuit 10,and is temporarily stored in the RAM.

When the above-mentioned operations are executed for one originalintroduced to the reading unit shown in FIG. 2, compressed image datarepresenting images on the front and back faces of the original isstored in the image buffer RAM 7.

After the input line of the input switching circuit 4 is switched to theimage data expansion circuit 6 side, the compressed image datatemporarily stored in the image buffer RAM 7 is read out by the imagedata transfer control circuit 10, and is supplied to the image dataexpansion circuit 6. The compressed image data is decoded by sequentialprocessing of the circuit 6. The decoded signal is subjected tore-encoding processing in the image data compression circuit 5 after thefirst half (i.e., front-face image data) or the second half (i.e.,back-face image data) in one scan line is selected by the front & backimage management circuit 12 via the input switching circuit 4. Theprocessing result is transferred to the SCSI bus transfer controlcircuit 8 under the control of the image data transfer control circuit10, and is then output onto the SCSI bus 9 under the management of thecontrol circuit 8.

With the above-mentioned operations, front & back batch compressed imagedata temporarily stored in the buffer RAM 7 for image data istemporarily decoded, the front- or back-face image data is selectivelyre-encoded in units of pages, and the re-encoded data is output onto theSCSI bus 9. Thus, the image reading apparatus which has the simple andcompact image reading unit, as shown in FIG. 2, and can selectively (orcontinuously) output front- and back-face images can be realized.

Since image data to be stored in the buffer RAM 7 for temporarilystoring front & back batch image data is compressed (encoded), anecessary memory capacity can be greatly decreased, thus reducing thecost of the apparatus.

Second Embodiment

FIG. 4 shows a circuit arrangement according to the second embodiment ofthe present invention. In this embodiment, two buffer RAMs 7a and 7b forstoring compressed image data are arranged, and an external storagedevice 11 is arranged on an SCSI bus 9. Thus, dual-face image data canbe continuously read, and a high-speed operation can be attained. Inaddition, since a plurality of dual-face batch image data can betemporarily stored in the external storage device 11, batch processingof a plurality of image data can also be performed. This embodiment willbe described in detail below.

The same reference numerals in FIG. 4 denote block components having thesame functions as in FIG. 1. In FIG. 4, the buffer RAM 7a for firstimage data, and the buffer RAM 7b for second image data have the samefunction, and the magnetic disk device 11 for temporarily storing imagedata is connected on the SCSI bus 9.

Image data of a first original read by two CCD image sensors 1a and 1bin the same sequence as in the first embodiment is temporarily stored inthe buffer RAM 7a for first image data, and image data of a secondoriginal is stored in the buffer RAM 7b for second image data. At thesame time, i.e., parallel to the storage operations, an image datatransfer control circuit 10 reads out image data stored first from thebuffer RAM 7a, and records the readout data in the magnetic disk device11 via the SCSI bus 9 under the control of an SCSI bus transfer controlcircuit 8.

The above-mentioned processing operations are repeated to operate thebuffer RAMs 7a and 7b in a double-buffer mode, and successively carrieddual-face originals can be sequentially recorded on the magnetic diskdevice 11 without stopping the originals.

After a required number of originals are recorded in the magnetic diskdevice 11 within the capacity of the device 11, the start page of theseries of originals temporarily stored in the magnetic disk device 11previously is read out, and is re-stored in the buffer RAM 7a or 7b.

The dual-face batch compressed image data re-stored in the buffer RAM 7aor 7b is decoded (expanded) by an image data expansion circuit 6 in thesame processing sequence as described in the embodiment shown in FIG. 1,and the decoded image is separated into front- and back-face image databy a front & back image management circuit 12. Thereafter, the separatedimage data are re-encoded (compressed) by an image data compressioncircuit 5, and the re-encoded data are output to, e.g., the magneticdisk device 11 on the SCSI bus 9. When the above-mentioned processingsequence is sequentially repeated, front- and back-face image data of aseries of previously read originals are separated and re-encoded, andthe re-encoded data can be continuously output to another device 11 onthe SCSI bus 9.

Therefore, in this embodiment, an original reading speed can beincreased as compared to the embodiment shown in FIG. 1, and after aplurality of originals are simultaneously and temporarily stored in thebuffer RAMs, they can be simultaneously output after front/backseparation.

In the series of descriptions associated with the embodiment of thepresent invention, front/back-separated image data is re-encoded(compressed), and is output to the external storage device 11 on theSCSI bus 9. However, binary image data may be output to an externaldevice without being re-encoded (compressed), or front & back batchencoded (compressed) data may be directly output to the external deviceif necessary.

As described above, the front and back faces of a dual-face original aresimultaneously read to be processed as one scan line, and the front- andback-face images are finally separated and output. Therefore, thefollowing effects can be obtained.

1 Since dual-face image data can be simultaneously read at substantiallythe same position, a dual-face image reading apparatus can have a simpleand compact arrangement.

2 Since front- and back-face image data of dual-face images can beprocessed as one scan line, a single electrical circuit necessary forimage processing need only be prepared. Therefore, a compact,inexpensive apparatus capable of performing high-speed readingprocessing can be realized.

3 Since front & back image data is finally separated, and front- orback-face image data is selectively output, an image output in the sameimage format as that of another conventional image processing apparatuscan be obtained.

In each of the above embodiments, after dual-face image data are readwithin one scan line, only one of front-face image data (i.e., the firsthalf of one scan line) and back-face image data (i.e., the second halfof one scan line) is selected as effective image data, and the selectedimage is stored in a file device.

Therefore, since single-face image data of the read dual-face image datais discarded, efficiency in terms of a reading speed of an originalimage is decreased to 1/2.

Thus, an arrangement for reading a single-face image without decreasingreading speed efficiency will be described below.

Third Embodiment

FIG. 5 shows a schematic circuit arrangement of principal part of animage reading apparatus according to the third embodiment of the presentinvention. CCD image sensors 1a and 1b simultaneously read front andback faces of a carried sheet-like original. A CCD drive control circuit32 performs drive control for operating these two CCD image sensors 1aand 1b. A mixing circuit 33 comprises an analog switch, and mixes analogpixel data from the two CCD image sensors 1a and 1b to obtain onecomposite signal. An image binarization circuit 34 A/D(analog-to-digital) converts the analog pixel data from the mixingcircuit 33 into digital multi-value pixel data, performs digital imageprocessing necessary for improving image quality, and then binarizes thepixel data using an internal digital comparator. An image datacompression/expansion circuit 35 performs compression by encoding animage signal in an original reading mode, and performs expansion bydecoding compressed data to an image signal in a reproduction outputmode.

A compressed image data file device 36 as one of the external devicesreceives and stores image data compressed by encoding by the image datacompression/expansion circuit 35. An image display 37 as anotherexternal device directly monitor-displays a read image in the readingmode, and displays a reproduced image searched from the compressed imagedata file device 36, and expanded by decoding by the image datacompression/expansion circuit 35 in the reproduction output mode.

An original feed motor 38 serves as a drive source for rotating thecarrying rollers 20a, 20b, 20c, and 20d shown in FIG. 2 so as tocontinuously carry sheet-like originals. An original feed motorcontrol/drive circuit 39 controls and drives the original feed motor 38.Fluorescent lamps 40a and 40b are light sources for respectivelyilluminating the front and back faces of an original to be read. Afluorescent lamp control circuit 41 for original illumination controlsthe light amounts of the fluorescent lamps 40a and 40b.

The CCD drive control circuit 32 switches a control operation mode inaccordance with a selection signal for instructing an effective readingface. This selection signal is normally supplied from a switch on anoperating panel 42, or a host device (not shown) such as a personalcomputer, and is a dual/single-face mode switching signal forinstructing a dual-face reading mode or an effective reading face (frontor back face) in a single-face reading mode. The selection signal isexpressed by two bits, e.g., 11, 01, or 10.

When the selection signal indicates the dual-face reading mode, the CCDdrive control circuit 32 performs operations as follows. Morespecifically, at this time, the CCD drive control circuit 32 outputssync signals to the two CCD image sensors 1a and 1b for a regulardual-face CCD drive period so that dual-face image data form one scanline, instructs a regular dual-face feed speed mode to the original feedmotor control/drive circuit 39, and instructs a half lighting mode (anillumination light amount half that in a full lighting mode) to thefluorescent lamp control circuit 41.

On the other hand, when the selection signal indicates the single-facereading mode, the CCD drive control circuit 32 performs uniqueoperations as follows. More specifically, at this time, the CCD drivecontrol circuit 32 selects, as an effective sensor, one of the CCD imagesensors 1a and 1b corresponding to an effective face indicated by theselection signal, and outputs a sync signal to the selected CCD sensorat a period twice the dual-face CCD drive period, so that single-faceimage data forms one scan line.

At the same time, the CCD drive control circuit 32 instructs a feedspeed twice the dual-face feed speed to the original feed motorcontrol/drive circuit 39 so as to obtain the same subscan resolution asin the dual-face reading mode, and to improve reading efficiency in thesingle-face reading mode. The circuit 32 instructs a full lighting modeto the fluorescent lamp control circuit 41 to obtain a light amounttwice that in the dual-face reading mode. In order to compensate for ahalved charge storage time of the CCD image sensor 1a or 1b incorrespondence with the doubled original feed speed, the originalillumination light amount is doubled. Thus, storage efficiency of thecompressed image data file device 36 can be prevented from beingimpaired, and at the same time, single-face reading efficiency can beimproved without degrading image quality.

As a light source for illuminating an original in an image readingapparatus of this type, a green fluorescent lamp suitable forsensitivity characteristics of a CCD image sensor is normally used.After a power switch is turned on, a time on the order of several tensof seconds to several minutes is required until the brightness of thefluorescent lamp is stabilized although it depends on the atmospherictemperature, and a longer time is required as the atmospherictemperature goes lower. This means that the apparatus cannot be usedsimply because of an insufficient light amount of the fluorescent lampfor illumination though all the other functions can be used. Meanwhile,in order to perform a twice-speed original carrying operation, the CCDimage sensor used in reading must be operated at a drive period twicethat in the dual-face reading mode (i.e., a half drive period). As aresult, since a storage time of the CCD image sensor is halved, theillumination light amount for illuminating an original must be doubledso as to obtain the same output value of the CCD image sensor as that inthe dual-face reading mode. This poses a serious problem in atwice-speed original carrying operation in consideration of the factthat characteristics of a stabilization curve of the brightness of thefluorescent lamp are expressed by an integral curve (e.g., a curverepresenting a voltage across C (capacitor) when a CR circuit ischarged).

In this embodiment, a third drive mode for operating the CCD imagesensor 1a or 1b used in reading at the same drive period as that in thedual-face reading mode before the fluorescent lamp is stabilized, isavailable, as will be described later. Thus, even in the single-faceoriginal reading mode, the apparatus can be operated in the same shortwait time as in the dual-face original reading mode. Furthermore, whenthe CCD drive control circuit 32 detects, based on a sensor output (notshown), that the light amount of the fluorescent lamp 40a or 40b hasreached a light amount necessary for a twice-speed original carryingoperation after an elapse of a certain time from power-on, itautomatically starts the twice-speed operation. Therefore, a single-faceoriginal can be very efficiently read immediately after the power switchis turned on.

FIGS. 6(A) to 8(F) show drive timings in the respective operation modeof the CCD image sensor according to the embodiment of the presentinvention. FIGS. 6(F), 7(F), and 8(F) show a correspondence between acomposite image and an actual image.

In FIGS. 6(A) to 6(F) showing the dual-face reading mode, a sync signalSH1 indicates an output start timing of a pixel string to the CCD imagesensor 1a used in reading of the front face. The CCD image sensor 1astarts outputting of a pixel string in response to a leading edge e1 ofthe signal SH1, and outputs an effective pixel signal for a section t1(FIGS. 6(A) and 6(C)). On the other hand, a sync signal SH2 for the CCDimage sensor 1b used in reading of the back face is output immediatelyafter the section t1 is ended. The CCD image sensor 1b starts outputtingof a pixel string in response to a leading edge e2 of the signal SH2,and outputs an effective pixel signal for a section t2 (FIGS. 6(B) and6(D)). A section t3 corresponds to a composite image output section inwhich the two effective signals are switched by the mixing circuit(analog switch) 33 shown in FIG. 5 near a switch point P1 between theeffective pixel output section t1 of the CCD image sensor 1a and theeffective period t2 of the CCD image sensor 1b, thus mixing the signalsas one composite image signal (FIG. 6(E)). When the front- and back-facesignals are mixed using the analog switch 33, one image processingsystem need only be prepared.

The above-mentioned timing control corresponds to a control contentperformed by the CCD drive control circuit 32 shown in FIG. 5 for theCCD image sensors 1a and 1b. At this time, the CCD drive control circuit32 supplies a regular speed instruction to the original feed motorcontrol/drive circuit 39, and supplies a half lighting instruction tothe fluorescent lamp control circuit 41.

The drive timing of the CCD image sensor when a single-face image (i.e.,an effective image only on the front face of an original) is to be readwill be described below. A twice-speed drive mode will be describedbelow with reference to FIGS. 7(A) to 7(F). In this case, after theeffective output section t1 of the CCD image sensor 1a is ended, no syncsignal SH2 for the CCD image sensor 1b is output (FIG. 7(B)), andinstead, the sync signal SH1 for the CCD image sensor 1a is successivelyoutput (FIG. 7(A)).

Therefore, a pixel string from the CCD image sensor 1a is output for asection t2' following the section t1 (FIGS. 7(C) and 7(D)). When thecontinuous pixel output from the CCD image sensor 1a is directly outputas a composite image output, a front-face image is output for two scanperiods during a composite image output section t3' (FIG. 7(E)). In thiscase, when a subscan speed is double, the same subscan resolution asthat in the dual-face reading mode can be obtained. Therefore, in thiscase, the CCD drive control circuit 32 shown in FIG. 5 performs theabove-mentioned timing control operation, supplies a twice-speedinstruction to the original feed motor control/drive circuit 39, andsupplies a full lighting instruction to the fluorescent lamp controlcircuit 41 since a storage time for photoelectric conversion of the CCDimage sensor is halved as compared to that in the dual-face readingmode.

A regular speed drive operation in the single-face image reading modewill be described below with reference to FIGS. 8(A) to 8(F). In thiscase, only the sync signal SH1 for the CCD image sensor 1a is output forthe same period as that in the dual-face simultaneous reading mode, andno sync signal SH2 for the CCD sensor 1b is output (FIGS. 8(A) and8(B)). Therefore, in this case, when the pixel output (FIG. 8(C)) fromthe CCD image sensor 1a is directly output as a composite image, afront-face image is output during the first half of a section t3", andthe second half of the section t3" corresponds to an ineffective sectionin which no image signal is output (FIG. 8(E)). In this case, the CCDdrive control circuit 32 shown in FIG. 5 supplies a regular speedinstruction to the original feed motor control/drive circuit 39, andsupplies a half lighting instruction to the fluorescent lamp controlcircuit 41.

As described above, since the image reading apparatus having threeoperation modes corresponding to a change in CCD drive method isconstituted, an optimal image reading operation can always be performedfor both dual- and single-face originals.

FIGS. 9 to 11 show schematic circuit arrangements according to thefourth to sixth embodiments of the present invention, and will bedescribed in turn hereinafter.

In the third embodiment shown in FIG. 5, in order to compensate for ahalf storage time for photoelectric conversion of the CCD due to adoubled original carrying speed, an original illumination light amountis doubled, while in the following three embodiments, the half storagetime is compensated for by changing parameters in an electrical circuit.For this reason, the following three embodiments are very effective whena CCD has a margin in its characteristics, e.g., sensitivity. Note thatthese four embodiments including the embodiment shown in FIG. 5 can besolely carried out, as a matter of course. However, compensation methodsin a plurality of embodiments may be combined.

The respective embodiments will be described below.

Fourth Embodiment

FIG. 9 shows a schematic circuit arrangement of a principal part of animage reading apparatus according to the fourth embodiment of thepresent invention.

In a single-face reading mode of this embodiment, in order to compensatefor a half storage time of a CCD due to a doubled original carryingspeed, a CCD drive control circuit 102 supplies a slice level switchingsignal to an image binarization circuit 104, thereby setting a slicelevel (threshold value) upon binarization to be a value half that in anormal mode (dual-face reading mode). Thus, the same density output asin the dual-face reading mode can be obtained.

Fifth Embodiment

FIG. 10 shows a schematic circuit arrangement of principal part of animage reading apparatus according to the fifth embodiment of the presentinvention. In a single-face reading mode of this embodiment, in order tocompensate for a half storage time of a CCD due to a doubled originalcarrying speed, a CCD drive control circuit 202 supplies a gainswitching signal to an amplifier 201a or 201b for amplifying an outputfrom a corresponding image sensor (these amplifiers are not shown inother embodiments although they are arranged), thereby doubling the gainof the corresponding amplifier as compared to a normal mode (dual-facereading mode). Thus, in both the single- and dual-face reading modes,the same amplified output voltage can be obtained for originals havingan identical density.

Sixth Embodiment

FIG. 11 shows a schematic circuit arrangement of principal part of animage reading apparatus according to the sixth embodiment of the presentinvention. In a single-face reading mode of this embodiment, in order tocompensate for a half storage time of a CCD due to a doubled originalcarrying speed, a sample·hold timing of a pixel signal instructed from aCCD drive control circuit 302 to an A/D converter 301 (which is notshown in other embodiments since it is included in the imagebinarization circuit 34) is delayed.

FIGS. 12(A) to 12(D) show change states of a CCD pixel signal outputduring one clock period in the sixth embodiment. As shown in FIG. 12(D),in a single-face reading mode (i.e., a storage time=1/2t), since achange amount of a pixel signal output voltage is halved as compared tothat in a dual-face reading mode (i.e., a storage time=1t), thesample·hold timing is changed to be delayed from that in the dual-facereading mode by a predetermined period of time, as shown in FIG. 12(D).Thus, substantially the same output voltage as that in the dual-facereading mode can be obtained.

In the above-mentioned embodiments, the CCD is used as the image sensor.However, the present invention can be applied to image sensors usingother solid state image pickup elements. The fluorescent lamp is used asa light source for original illumination. However, the present inventionis not limited to this. For example, the present invention can beapplied to, e.g., a xenon lamp. The present invention can be applied notonly to a copying machine or a facsimile apparatus in which an imagereading unit and a recording unit are integrated, but also to an inputterminal device for, e.g., a computer.

As described above, in a dual-face image reading apparatus which cancontinuously and simultaneously read front- and back-face images of anoriginal using a pair of image sensors within one scan line, the driveperiod of the image sensor is doubled in the single-face originalreading mode so as to double the original carrying speed, and a meansfor compensating for a half storage time of the image sensor due to thedoubled carrying speed is arranged. Therefore, an image readingapparatus which can obtain optimal reading performance for both dual-and single-face originals without impairing image quality and readingefficiency can be realized.

When a light source for original illumination requires a considerabletime until a predetermined light amount is reached, substantially thesame drive control as in the dual-face reading mode (except that onlythe corresponding image sensor is selected as an effective sensor) isperformed in the single-face reading mode. Therefore, a wait time can beshortened, and a good image can be obtained.

Some preferred embodiments of the present invention have been described.However, the present invention is not limited to the arrangements ofthese embodiments, and various changes and modifications may be madewithin the scope of claims.

What is claimed is:
 1. An image processing apparatus comprising:firstinput means for inputting first image data representing an image on afront face of an original line by line; second input means for inputtingsecond image data representing an image on a back face of the originalline by line; mixing means for mixing the first image data and thesecond image data line by line and for outputting mixed image data lineby line; instruction means for instructing one of the front and backfaces of the original; selection means for selecting one of the firstand second image data among the mixed image data output from said mixingmeans line by line in accordance with the instruction by saidinstruction means; temporary memory means for temporarily storing themixed image data output from said mixing means; compression means forcompressing the mixed image data output from said mixing means, whereinsaid temporary memory means stores the mixed image data compressed bysaid compression means; and expansion means for expanding the compressedmixed image data read out from said temporary memory means, wherein saidselection means selects one of the first and second image data among themixed image data expanded by said expansion means.
 2. An apparatusaccording to claim 1, wherein said compression mean compresses one ofthe first and second image data selected by said selecting means.
 3. Anapparatus according the claim 1, further comprising storage means forstoring the image data selected by said selecting means.
 4. An apparatusaccording to claim 1, wherein said first and second input means comprisefirst reading means for reading the image on the front face of theoriginal and second reading means for reading the image on the back faceof the original, respectively.
 5. An image processing apparatuscomprising:first reading means for reading an image on a front face ofan original and for supplying first image data line by line; secondreading means for reading an image on a back face of the original andfor supplying second image data line by line; mixing means for mixingthe first image data and the second image data and for outputting mixedimage data line by line; compression means for compressing the mixedimage data line by line; temporary memory means for temporarily storingthe mixed image data compressed by said compression means; expansionmeans for expanding the compressed mixed image data read out from saidtemporary memory means line by line; and selecting means for selectingone of the first and second image data among the mixed image dataexpanded by said expansion means.
 6. An apparatus according to claim 5,wherein said first and second reading means simultaneously read theimages on the front and back faces of the original.
 7. An apparatusaccording to claim 5, wherein said compression means compresses one ofthe first and second image data selected by said selecting means.
 8. Anapparatus according to claim 5, further comprising instruction means forinstructing one of the front and back faces of the original, whereinsaid selecting means elects one of the first and second image data inaccordance with the instruction by said instruction means.
 9. Anapparatus according to claim 5, further comprising storage means forstoring the image data selected by said selecting means.